Scientists in the Electronics
Science and Technology Division at the Naval Research Laboratory
(NRL) have developed a way to measure, with great clarity, individual
quantum dots formed of the semiconductor gallium arsenide, with
diameters of about 10 nanometers. A quantum dot is a crystal
structure so small that its internal energy can only take on
certain discrete values and is often described as the solid-state
analog of an atom.
Current research on quantum dots
is largely a continuation of work on higher dimensional quantum
wells and quantum wires -- work that began in the 1970s and continues
today. Recently, considerable research has been conducted on
the optical properties of quantum dots. The current surge is
fueled both by the continuing anticipation that quantum dots
will improve the performance of devices such as lasers, electro-optical
devices and optical memories, and recent improvements in growth
techniques. Unfortunately, all current types of quantum dots
suffer from relatively large fluctuations in size. Size fluctuations
lead to spectral line broadening, known as inhomogeneous broadening,
which blurs the spectra and severely limits the information one
can obtain from optical spectroscopy. This severe inhomogeneous
broadening problem has motivated the development of a new research
field -- that of single quantum dot spectroscopy. Within the
last three years, there has been an explosion of effort to resolve
and study individual quantum dots optically.
NRL recently sponsored (July
21 and 22, 1997) a workshop entitled, "Recent Advances in
the Physics of Single Quantum Dots," which brought together
over 80 scientists who are active or interested in this new field.
Single quantum dots are resolved spatially and spectrally using
nanoscopic spectroscopic techniques such as optical near-field
spectroscopy, sensitive detectors, and resonant laser excitation.
By measuring the spectra of individual quantum dots instead of
ensembles, it is possible to sharpen the spectra enormously.
Decreases in spectral line widths of over two orders of magnitude
have been recorded. Such extraordinary improvements in resolution
make possible the observation of a number of phenomena for the
first time.
In 1996, the NRL researchers,
led by Dr. Daniel Gammon of the Electronic Materials Branch,
reported the discovery of an unexpected fine structure in the
spectral lines that they have interpreted as a splitting of the
spin degeneracy. They went on to report hyperfine shifts in the
spectral lines due to interactions between the spin of the electrons
with the spins of the underlying nuclei. These discoveries again
are reminiscent of the early days of atomic spectroscopy at the
turn of the last century as improvements in equipment and techniques
led to the measurement of fine and hyperfine structure in the
spectra of atomic gases. All previous measurements in single
quantum dots have measured the electronic spectra. However, the
electronic properties of quantum dots, and semiconductors in
general, represent only part of the picture. The nuclei that
make up the underlying lattice of the quantum dot are also important.
Unfortunately, conventional nuclear spectroscopy is much less
sensitive than the optical spectroscopy whereby the electronic
properties are usually probed.
However, now, in a paper that
has just been published in the journal Science, the NRL
group reports the demonstration of nuclear spectroscopy of individual
quantum dots. Both the nuclear spin and the nuclear vibrations
were measured at the single quantum dot level in novel examples
of Raman and nuclear magnetic resonance spectroscopies. These
very challenging experiments represent an improvement in sensitivity
of five orders of magnitude over previous semiconductor measurements.
Such nanospectroscopy opens up the possibility of measuring lattice
properties, such as strain and composition, on the 10-nanometer
scale with a spectral sharpness that is well beyond ensemble
measurements. According to Dr. Gammon, "NRL and the Navy
anticipate that advanced nanostructured materials, such as quantum
dots, will lead to improvements in a variety of technologies
ranging from communications to remote sensing."
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